In recent years, solid-state image sensors have been widely used at the imaging surface of video cameras, still cameras, and the like. These solid-state image sensors have been miniaturized as technology has advanced. Accompanying this trend, there has been an increased demand for a more compact photographic lens having a wider viewing angle than previously available.
Recently, there has been a large increase in demand for digital still cameras, and various data types are now frequently put into a photographic record. Distortion resulting from the photographic lens is quite conspicuous in this sort of photographic recording of data where the photographic subject data comprises, for example, the wording of a business card or a table format. This is because these subjects generally include an image that is rectangular in shape and is located near the periphery of the image, as results from photographing the outer dimensions of a business card, the framework of a table, or the like. Accordingly, it is strongly desired that the distortion of a photographic lens used in this type of camera be kept small. In order to realize a wide image angle, it is advantageous to use an imaging lens of the retro focus type. However, in an imaging lens of the retro focus type, it is necessary to correct the various aberrations that are generated by the negative lens element having a large aperture that is arranged on the object side.
In order to accomplish this, it is thought that, by arranging a convex lens element with low refracting power on the extreme object side in an imaging lens of the retro focus type, the distortion will be corrected. An example of this approach, which uses a biconvex lens element arranged on the extreme object side, is Japanese laid open Patent Publication No. H09-80302. However, the imaging lens is insufficient in that its f-number is too large (2.8) to yield a bright image and, in addition, the width of the image angle (i.e., the field of view) and distortion certainly could not be said to be desirable.